Especially his philosophy of logic and sense/reference (a continuation of the work of Frege and Russell) is mostly forgotten. He published many papers on this topic, yet they aren't discussed e.g. in Wikipedia. At least the SEP entry is better:

https://plato.stanford.edu/entries/church/

Though I heard that it also neglects some major parts of his work. I assume it was too philosophical for mathematicians and too technical for philosophers.